Hydrogen cryomagnetics is a term used to denote the use of cryogenic liquid hydrogen to cool the windings of an electromagnet. A key benefit of hydrogen cryomagnetics is that low temperature liquid hydrogen can be deployed simultaneously both as a cryogen to cool electromagnet windings and as an energy carrier . That is, powerful synergistic benefits are likely to arise when hydrogen is used as a fuel and as a coolant. Even without the fuel/coolant synergies, hydrogen cryomagnetics is an attractive option for the cooling of superconducting electromagnets as it eliminates dependence upon increasingly scarce and expensive liquid helium. For hydrogen cryomagnetic applications specialist hydrogen-cooled electromagnets are wound using either copper or superconductors. Liquid-hydrogen-cooled copper-wound magnets work well as pulsed field magnets. Superconductors have the property that they can operate continuously and very efficiently as electrical resistive losses are almost entirely avoided. Most commonly the term "hydrogen cryomagnetics" is used to denote the use of cryogenic liquid hydrogen directly, or indirectly, to enable high temperature superconductivity in electromagnet windings. Hydrogen cryomagnetics is especially useful where high magnetic fields are required, such as in high torque electric motors. At standard atmospheric pressure liquid hydrogen boils at approximately 20.3 K (-259.3 °C). Liquid hydrogen at such a temperature is significantly colder than the temperatures at which superconductivity can first be induced in a range of important high temperature superconductors including yttrium barium copper oxide (YBCO), because YBCO has a superconducting transition temperature (Tc) of 93 K. The operation of YBCO-based superconducting magnets at a temperature more than 70 K below Tc allows for the use of very high current densities and very high magnetic fields without loss of superconductivity. The materials properties of YBCO are such that it cannot be made into ductile wires although much progress has been made towards high field YBCO electromagnets based on the use of tapes rather than wires. Another superconductor suitable for hydrogen cryomagnetic use is magnesium diboride. Magnesium diboride is a conventional superconductor and it can be prepared in flexible wires facilitating its potential application in, for example, tokamak fusion reactors. Magnesium diboride has a transition temperature of 39 K. While at atmospheric pressure liquid hydrogen is cold enough to cool magnesium diboride into the superconducting state, there are advantages to pumping on the hydrogen so as to lower its temperature still further when in use such a magnet winding (this uses the same physics that says that the boiling point of water can be reduced by reducing the pressure above the liquid, see e.g.). Generally the greater the difference between conductor temperature and superconducting transition temperature the better. Liquid hydrogen is not the only way cryogenically to cool a magnet, indeed conventionally superconductors are cooled using liquid helium at 4.2K and for conventional conductor pulsed magnets (including copper) most attention has been given to liquid nitrogen at 77 K. Liquid hydrogen can be expected to drive better performance than liquid nitrogen and, as discussed below, liquid hydrogen avoids several concerns around helium availability. Any use of hydrogen cryomagnetics requires careful consideration of hydrogen safety. Hydrogen cryomagnetics is concept distinct from the use of higher temperature gaseous hydrogen as a coolant in power plant turbines.
Origins The term hydrogen cryomagnetics was first used in a text panel forming part of an article by Professor WJ Nuttall and Professor BA Glowacki published in July 2008 in Nuclear Engineering International. The concept was returned to in an Institute of Physics conference held in Manchester England in April 2010. The presentation was delivered by Professor WJ Nuttall and co-authored by Professor BA Glowacki and Dr L Bromberg. The journey to the term also involved thinking around Hydrogen as a Fuel and as a Coolant – from the superconductivity perspective. Earlier related consideration of liquid hydrogen as a cryogenic coolant includes work by Glowacki and co-authors from 2005 and 2006. The concept of hydrogen cryomagnetics has been further elaborated and discussed in 2012, 2015 and 2019.
Attributes The emergence of hydrogen cryomagnetics can be expected to benefit from the development of strong industrial interest in liquid hydrogen that can be expected to occur for other reasons, including the growth of a general hydrogen economy and the need to transport and store bulk hydrogen. Global interest is growing in the emergence of a hydrogen economy in which hydrogen is a low-carbon energy carrier sourced from renewables (green hydrogen) or alternatively from natural gas with carbon capture and storage (this is sometimes termed "blue hydrogen"). When pipelines are unavailable. the use of liquefied hydrogen for the bulk transport and distribution of hydrogen molecules has been found to be the more efficient than high pressure gas cylinders when moving the large quantities over the large distances. Hydrogen (as liquid or gas) is an energy storage system in competition with electric battery technology. Hydrogen wins out over batteries for the largest quantitites of energy stored over the longest period. Hydrogen fuel cells are win out over battery electric technologies for the heaviest forms of transportation - such as trains, trucks and buses Hydrogen technology is in competition with battery technology and gaseous hydrogen technology is in competition with liquid hydrogen technology. As these competitive forces pay out it is quite possible that a significant role will emerge for liquid hydrogen as a stationary long-term and large-scale energy storage system and fuelling system for heavier vehicles. In such scenarios, the emerging economic role of liquid hydrogen production and distribution can be expected to greatly favour the subsequent use of hydrogen in cryomagnetic applications.
Avoiding the problems of helium
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